MPP Mk8 not focusing right

[Ric Bower]

Hi- any advice would be appreciated on this subject. I have what I think is an MPP Mk 8 and it consistantly focuses 1.5" further away than it should if the subject distance is 6ft. I have tested using a newspaper pinned to a board at 45 degrees to the plane of focus and marked clearly the intended plane of focus on the paper- spotting the actual focus is easy with the finely serifed text. I only use 150mm symmars and usualy wide open for location portraiture so this is obviously a real problem. I use Toyo darkslides and focus with a powerful loupe.
Thanks in advance
Ric

[John_4185]

The problem could be the film holder. Toyo... well, my confidence is not with them. Try a different film holder, a Fidelity Elite for example.

[Steve Hamley]

jj,

What experience do you have with Toyos causing you to be "unconfidant"? Have you measured them? I'm curious since I have some 8x10s.

Steve

[John_4185]

Steve: I leave it up to you. I am skeptical of ALL holders. If you are in the US, then here's the simple test: stack three brand new US Quarters inside a film holder with the slide removed. Put a straightedge across the top. If the top of the stack is not even with the filmholder's side surfaces, then the holder is not good enough. IMHO! Leave it to the US Mint to make vast quantities of cheap thingies to greater precision than certain LF film holder makers.

[Michael Gudzinowicz]

Ric,

If you use the simple lens equation 1/focal_length = 1/image_dist + 1/object_dist,
you'll find that the film plane is 0.297 mm closer to the lens than the point of focus
using the loupe.

You might want to check the following:

1. Does your high power loupe actually focus on the ground glass surface, or is focus
displaced 0.3 mm back from the surface? Some GG panels have a clear or fine center
spot to view an aerial image which can lead to an error if loupe focus isn't perfect. If
this is the case, adjust the loupe focus 0.3 mm closer to the lens.

2. Check the film holders with a micrometer depth gauge for consistancy using a sheet
of film with the holder held vertically. If the holders are consistent, compare the distance
to depth of the GG in its frame, and check that the focusing panel makes even contact
with the registration surface of the back in contact with film holders.

If the holders are consistent and within the ANSI spec, consider moving the GG 0.3 mm
forward if possible. If someone has repainted the GG frame or put light-trap fabric on the
GG frame, remove it. If not, shim the GG by 0.3 mm if the GG is inserted from the front
the frame. If those approachs don't reveal the problem and the holders are out of the
ANSI spec by 0.3 mm, you might consider replacing the holders.

3. Stop the lens down to f/11 to f/16 which will give limited depth of field but will permit
sharp (front to rear) 16x20 portraits if the focus error persists.

Mike

[Colin Carron]

Ric,

Have a look on the MPP Users Club website for info about register distances :

http://www.mppusers.freeuk.com/registers.htm

I have an MPP Mk VII and use MPP, Fidelity and Toyo slides which all seem to be OK .

[robc]

everyone works to their own level of precision. If you want to work to the highest level of precision then its not safe to rely on someone elses level of precision. The moral being; Measure the depths yourself and don't rely on quoted numbers of what something is supposed to be. Often things turn out to be different than expected. For example; is your lens really 150mm? If its a schneider APO Symmar 150 then it may be 151.5mm focal length thereby altering numbers. OK the difference is thousandths of a mm in focus depth but all these little numbers add up and when they all happen to fall in the same direction then the perceived error can become large.
I agree with mikes numbers excepting I used 151.5mm focal length which makes it 0.3mm.
However, we are relying on your meausrements being 100% precise. i.e. is the object distance really 6ft or is that to within an inch or two. And is the error exactly1.5 inches. How did you measure these accurately?

TOYO Holders:
There is nothing wrong with them. It just happens that they may not conform to someone elses idea of what the standard should be. Who cares. If you have all TOYO holders and they are all the same depth and your GG is set for that depth then they will be just as good as any other film holder. (cheaper too)

[robc]

JJ,
Steves question related to 8x10 holders. I think your method relates to 4x5 holders.

[Ric Bower]

Thank you all for the quality and speed of your replies- Here is what i have wound up doing as a stop gap since i have a job with camera and lens combo on Monday. I made a mark on the measuring newspaper where it was crisp 1.5 inches back from the point where had focused previously, the actual plane of focus established by my test prints, I then focused carefully on this point. Next I shimmed the ground glass screen using strips of 4x5 film until the previous focal mark 1.5" further back came into focus. I have just shot some test sheets to see if it has worked!
regards
Ric

[Ric Bower]

I got the direction wrong, shimming made the problem worse meaning the ground glass needs to be sunk into the frame by a fraction of a mil- is grinding paste a viable option here? would a fresnel installed lens side have the desired effect, if so is there any particular type and how would i best go about it? I think I will stop down for the job on Monday!
Thanks again
Ric